1. Field of the Invention
The present invention relates to a CMOS current source circuit, and particularly to an improved CMOS current source circuit capable of constantly generating a certain reference voltage irrespective of an analog supplying voltage, a substrate temperature, and a temperature variation.
2. Description of the Conventional Art
Generally, in a high speed memory construction, an analog circuit such as a DLL (delay-locked loop) is adopted in order to reduce an access time of the memory. Here, DLL is subjected to a temperature T or a supplying voltage Vdd. Therefore, a current source circuit capable of constantly generating a certain reference current Iref irrespective of the above-mentioned factors is necessary.
FIG. 1 shows a conventional current source circuit, which includes PMOS transistors MP1, MP4, and MP5, PMOS transistors MP2 and MP3, and NMOS transistors MN3 and MN4, each of which is formed with a current mirror.
To begin with, an analog voltage Vdda is supplied to the current source circuit as shown in FIG. 1. In this state, a temperature T is increased, the current I1 can be obtained in accordance with the following expression, when a resistance R1 is applied to the base-emitter Vbe2. EQU I1=Vbe2/R1 formula 1
Here, the current I1 of the formula 1 is in inverse proportion to temperature because the same is decreased by -2 mV/.degree. C.
In addition, the current I2 is caused when the difference between the base-emitter voltage Vbe2 of the bipolar transistor Q2 and the base-emitter voltage Vbe1 of the bipolar transistor Q1 are applied to the resistance R2. That is, the current I2 is obtained as follows. EQU I2=(Vbe2-Vbe1)/R2=nT/R2 formula 2
where n denotes a constant irrespective of temperature.
Therefore, the current I2 is in proportion to the temperature increase, and when the NMOS transistor MN4 has the same ratio of "width(w)/length(l)" as the NMOS transistor MN3, the current I3 is the same as the current I2.
In addition, since the PMOS transistors MP1, MP4, and MP5 is formed with a current mirror, the current I3 flows through the PMOS transistor MP1. In addition, since the PMOS transistors MP2 and MP3 are formed with a current mirror, the current I1 flows through the PMOS transistor MP2.
Here, the bias current Ibias is obtained by adding the current I1 and the current I3. That is, it is obtained by the following expression. EQU Ibias=I1+I3=Vbe2/R1+nT/R2 formula 3
Therefore, when temperature T is increased, since the bias current Ibias is the sum between the current I1 which is decreased in accordance with the increase of the temperature T and the current I2 which is increased in accordance with the decrease of the temperature T, the bias current is constant.
However, since the conventional current source circuit adopts the bipolar transistor which has the emitter of the P.sup.+ diffusion layer, the base of n-well, and the collector of the P.sup.- substrate in a n-well formation process in order to generate a constant bias current Ibias, substrate currents are generated.
Therefore, this substrate currents cause variation of substrate voltage in accordance with an internal resistance component, and the substrate voltage varies the threshold voltage Vt, so that the bipolar transistor characteristics are varied, and analog devices which require a constant substrate voltage may be affected by the above-mentioned variations.